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Distinguishing Palmer amaranth and waterhemp

The lack of reliable traits to distinguish Palmer amaranth and waterhemp during vegetative stages complicates efforts at stopping the spread of Palmer amaranth across Iowa.

However, both plants should be in full reproductive mode at this time, greatly simplifying the identification of the two amaranths.

While most agronomists and weed scientists prefer to identify weeds using vegetative traits, the small bracts (modified leaves) associated with flowers of Palmer amaranth and waterhemp are the most, if not only, reliable way to differentiate the two species.

Palmer amaranth has relatively large, green bracts that extend well beyond the other flower parts, whereas on waterhemp the bracts are similar in length to the tepals surrounding the seed capsule. On close examination, Palmer amaranth’s bracts on mature female plants are easily seen protruding from the plant’s seedheads without the use of a hand lens. Redroot and smooth pigweed also have large bracts; however, these species have hairy stems in contrast to the smooth stems of Palmer amaranth and waterhemp.

Several people from across the state have submitted photos or samples of plants suspected of being Palmer amaranth. In all but one case they were simply ‘healthy’ waterhemp. The exception was from Lee County in the SE corner of Iowa. This brings the number of counties with confirmed infestations of Palmer amaranth to five: Fremont, Harrison, Lee, Muscatine and Page.

Now is the easiest time to find new infestations of Palmer amaranth and initiate programs to either eradicate or limit its spread. We appreciate being informed of new Palmer amaranth infestations and are willing to aid in identifying suspect plants.